"I'm just enjoying this," said McDonald, who had never won an ATP Tour-level match before or earned an ATP ranking point. "I'm 18 years old. I'm playing a Masters-1000 main draw that I qualified for, so it's pretty unique."

McDonald, who plans to enter UCLA next month, reached the Australian Open boys' semifinals in 2012 but has done little in the last few months, even on the junior level, to suggest such a breakthrough.

Four days ago, he lost in the round-of-16 of the boys' 18 national championships at Kalamazoo, Mich. He said he had geared his season toward winning the prestigious event, where the champion is traditionally awarded a main draw wild card into the U.S. Open.

"I definitely felt more pressure out there, because a U.S. Open wild card is at stake, and I really thought I had a great chance to win it," said McDonald, who was seeded No. 9. "I did get tight at times there."

As he departed the tournament, Jay Berger of the USTA men's player development program offered him a qualifying wild card in Cincinnati. He actually contemplated leaving it on the table to play a college event in Indiana but decided to take his chance.

McDonald, a rangy player with solid groundstrokes and a fluid, potent serve struggled to explain his success except to say he was serving better than ever and liked the added pace from the bigger hitters on the pro tour.

He was also more relaxed. Deep in the third set against Johnson, the 2011-12 NCAA singles champion from USC, McDonald repeatedly glanced over at his box in the 5,000-seat Grandstand court and broke into a deep smile.

"I'm having a blast out here, for sure," he said. I mean for me, there's a lot less pressure, maybe, than some of the other guys. I don't have much to lose."

McDonald, who finished his last two years of high school via online courses, has been working with former top-10 player Wayne Ferreira of South Africa in the San Francisco Bay Area since he was 11.

He called the decision to delay turning pro and attend college a tough one, but with extensive ties to UCLA — his father and grandfather are alumni and his sister is currently a sophomore — he felt it was a good one.

"There is a lot to gain from UCLA," said the 5-10 McDonald, who described himself as weighing 142 pounds "on a good day."

"I can get stronger. I can learn a lot."

Before taking the court, McDonald said he called the UCLA coach, Billy Martin, for advice, and made sure to wear his Bruins cap.

Unless he wins the tournament — a suggestion that made McDonald's eyes go wide — he will stick with his plan become a Bruin this fall.

But having already beaten a double college champion in Johnson — from rival USC no less — his college prospects appear extremely promising.

This weekend showed McDonald could be making noise in the pros soon, too.